The movement of water between blood, brain and cerebrospinal fluid will be studied. The effect of changes of serum and spinal fluid osmolarity on the cerebrospinal fluid formation rate will be measured by the technique of ventricular perfusion. Since total spinal fluid originates only in part from the choroid plexus an attempt will be made to demonstrate that under the conditions of progressive hypotonicity of the blood the source of an increase in the spinal fluid formation rate is from the brain. The movement of fluid through the extracellular space could represent a mechanism by which water could "drain off" or "overflow" into the cerebrospinal fluid compartment. In this way an excess of fluid accumulated by the brain would be limited. The mechanism of how intraventricular pressure influences cerebrospinal fluid dynamics wll be studied in both normal and experimentally induced, obstructed hydrocephalic cats. In these animals, the effect of increased intraventricular pressure on regional cerebral blood flow will be examined in order to explain how spinal fluid formation may be altered under these conditions. The means by which alternate spinal fluid absorptive pathways are developed and maintained will be examined in hydrocephalic cats with respect to changes in blood in the periventricular white matter. Regional blood flow in the brain will be determined from the product of the fractional uptake of a diffusible indicator substance and the cardiac output measured in the same animal by an indicator dilution method.